1. Field of the Invention
This invention relates generally to a system and method for monitoring the cell voltages of fuel cells in a fuel cell stack and, more particularly, to a system and method for monitoring the cell voltages of fuel cells in a fuel cell stack that includes providing calibration pulses before cell voltage measurement pulses in a modulated cell voltage signal, where the calibration pulses have a start of frame sequence defined by a high voltage—a low voltage—a high voltage—a low voltage pattern that will not be reproduced by normal cell voltage measurements.
2. Discussion of the Related Art
Hydrogen is a very attractive fuel because it is clean and can be used to efficiently produce electricity in a fuel cell. A hydrogen fuel cell is an electro-chemical device that includes an anode and a cathode with an electrolyte therebetween. The anode receives hydrogen gas and the cathode receives oxygen or air. The hydrogen gas is dissociated in the anode to generate free protons and electrons. The protons pass through the electrolyte to the cathode. The protons react with the oxygen and the electrons in the cathode to generate water. The electrons from the anode cannot pass through the electrolyte, and thus are directed through a load to perform work before being sent to the cathode.
Proton exchange membrane fuel cells (PEMFC) are a popular fuel cell for vehicles. The PEMFC generally includes a solid polymer electrolyte proton conducting membrane, such as a perfluorosulfonic acid membrane. The anode and cathode typically include finely divided catalytic particles, usually platinum (Pt), supported on carbon particles and mixed with an ionomer. The catalytic mixture is deposited on opposing sides of the membrane. The combination of the anode catalytic mixture, the cathode catalytic mixture and the membrane define a membrane electrode assembly (MEA). MEAs are relatively expensive to manufacture and require certain conditions for effective operation.
Several fuel cells are typically combined in a fuel cell stack by serial coupling to generate the desired power. For example, a typical fuel cell stack for a vehicle may have two hundred or more stacked fuel cells. The fuel cell stack receives a cathode input reactant gas, typically a flow of air forced through the stack by a compressor. Not all of the oxygen is consumed by the stack and some of the air is output as a cathode exhaust gas that may include water as a stack by-product. The fuel cell stack also receives an anode hydrogen reactant gas that flows into the anode side of the stack. The stack also includes flow channels through which a cooling fluid flows.
The fuel cell stack includes a series of bipolar plates positioned between the several MEAs in the stack, where the bipolar plates and the MEAs are positioned between the two end plates. The bipolar plates include an anode side and a cathode side for adjacent fuel cells in the stack. Anode gas flow channels are provided on the anode side of the bipolar plates that allow the anode reactant gas to flow to the respective MEA. Cathode gas flow channels are provided on the cathode side of the bipolar plates that allow the cathode reactant gas to flow to the respective MEA. One end plate includes anode gas flow channels, and the other end plate includes cathode gas flow channels. The bipolar plates and end plates are made of a conductive material, such as stainless steel or a conductive composite. The end plates conduct the electricity generated by the fuel cells out of the stack. The bipolar plates also include flow channels through which a cooling fluid flows.
As a fuel cell stack ages, the performance of the individual cells in the stack degrade differently as a result of various factors. There are different causes of low performing cells, such as cell flooding, loss of catalyst, etc., some temporary and some permanent, some requiring maintenance, and some requiring stack replacement to exchange those low performing cells. Although the fuel cells are electrically coupled in series, the voltage of each cell when a load is coupled across the stack decreases differently where those cells that are low performing have lower voltages. Thus, it is necessary to monitor the cell voltages of the fuel cells in a stack to ensure that the voltages of the cells do not drop below a predetermined threshold voltage to prevent cell voltage polarity reversal, possibly causing permanent damage to the cell.
Typically, the voltage output of every fuel cell in a fuel cell stack is monitored so that the system knows if a fuel cell voltage is too low, indicating a possible failure. As is understood in the art, because all of the fuel cells are electrically coupled in series, if one fuel cell in the stack fails, then the entire stack will fail. Certain remedial actions can be taken for a failing fuel cell as a temporary solution until the fuel cell vehicle can be serviced, such as increasing the flow of hydrogen and/or increasing the cathode stoichiometry.
Fuel cell voltages are often measured by a cell voltage monitoring sub-system that includes an electrical connection to each bipolar plate, or some number of bipolar plates, in the stack and end plates of the stack to measure a voltage potential between the positive and negative sides of each cell. Therefore, a 400 cell stack may include 401 wires connected to the stack. Because of the size of the parts, the tolerances of the parts, the number of the parts, etc., it may be impractical to provide a physical connection to every bipolar plate in a stack with this many fuel cells, and the number of parts increases the cost and reduces the reliability of the system.
As discussed above, it is known in the art to process the voltages of the fuel cells in a fuel cell stack to determine whether the fuel cell stack is functioning as desired. Sometimes, cell voltage processing is performed every other cell because of the costs associated with monitoring every cell. Furthermore, it can be difficult to provide the necessary components in the space available to monitor every cell. In order to eliminate the need to connect fuel cell measurement circuits to a fuel cell stack using a plurality of interconnecting wires, it is desirable to embed such measurement circuits directly within the structure of the fuel cell stack assembly. Such an embedded measurement circuit would not add significant cost, and would allow for every fuel cell to be monitored.
U.S. patent application Ser. No. 12/840,047, titled Stack-Powered Fuel Cell Monitoring Device With Prioritized Arbitration, filed Jul. 20, 2010, assigned to the assignee of this application and herein incorporated by a reference, discloses a system and method for monitoring the cell voltages of fuel cells in a fuel cell stack. The system includes a plurality of voltage sensors coupled to the fuel cells in a fuel cell group, and a plurality of oscillators, where a separate oscillator is coupled to each of the sensors. Each oscillator operates at a different frequency, where higher frequency oscillators are coupled to lower priority sensors and lower frequency oscillators are coupled to higher priority sensors. The system also includes a light source, such as an LED, that receives frequency signals from the oscillators, where the light source switches on and off in response to the frequency signals and where lower frequency signals dominate the switching of the light source. A light pipe receives the switched light signals from the light source and provides light signals at a certain frequency at an end of the light pipe. A photodetector detects the light signals at the end of the light pipe.
The system disclosed in the '047 application for monitoring cell voltage has limitations in accurately providing the cell voltage, as do most other known cell voltage monitoring systems. As the fuel cell industry progresses, it is desirable to provide at least a 10-15 mV resolution accuracy of the voltages measured from the fuel cells. So far, this level of accuracy has been difficult to achieve using standard automotive sensors and parts. If the cell voltages are not accurately represented, then control of the various fuel cell system operations is limited in its accuracy, which could result in lower system performance, inefficiencies, cell degradation, etc. Further, prior cell voltage monitoring systems typically allowed the monitoring device to identify a minimum cell voltage, a maximum cell voltage and an average cell voltage, but were unable to identify which voltage was associated with which cell. It would be advantages to have this information so that a technician can identify a specific cell that may be failing.